The next stage after the removal of the
solids is the reduction of dissolved waste such as ammonia and nitrite. The
biological filter is an area devoted to the support and concentration of
bacteria needed to break down the waste.
The waste from the fish is the
start of the nitrogen cycle in the pond this is an important biological cycle
that nature uses to recycle the waste that the fish produce.
The figure shown is a very simplified one and shows the
basic actions in the cycle.
The start is when the fish are fed with food,
they covert it in to body mass and use some as energy to live, the waste is
excreted in the form of ammonia. The naturally occurring bacteria in the pond,
nitrosomonas, break ammonia down to nitrite using oxygen. Once this has been
done the next set of bacteria, nirtobacter, get to work converting nitrite to
nitrate, this process also requires oxygen.
The final product nitrate is
plant food and the pond plants good and bad(algae) need this to grow and will
consume as much as they can. The nitrate level will rise as it is unlikely that
the plants in the pond will be able to keep up with the nitrate
What Bacteria Want.
The bacteria have a few simple requirements , the
first being oxygen, this is needed to break down ammonia to nitrite and nitrite
to nitrate.The food required is readily supplied by the fish in the form of
The important issues that effect biological filters are :
- Oxygen supply.
With out oxygen the bacteria will die and
anaerobic bacteria will take over and convert nitrate back to nitrite and
poison the fish. The levels need to be as high as possible. The addition of air
stones to the filter will help the process.
The higher the temperature the better it will work,
until the oxygen level drops off at very high temperatures.
Many treatments will effect the filters efficiency by
either reducing the oxygen levels or by directly killing the bacteria.
- Water flow.
As the water brings both the food and oxygen, this is
a very important factor. A good flow rate to aim for is 8 to 10 times the
volume of the bio-filter per hour, the filter will work at much higher speeds
but this is the ideal rate.
The filter media.
The filter media is the bacteria's home and should offer a large surface
area as the more bacteria you can support the more work the filter can achieve.
The media needs to be rough so that the bacteria can grow on the surface. It
must have a good structure so that water flows evenly through it so that no
dead areas can become anaerobic (oxygen deficient).
There are a huge variation of medias on the market today ranging from lava
rock to plastic sheeting.
All need to fill the basic needs of a good filter
media, they should compile with all of them.
- They should be non toxic and contain no recycled material ( as it is
difficult to confirm the content of a recycled product.)
- Media should have a large surface to volume ratio. The more bacteria the
filter can hold the smaller the filter can be.
- The media should contain no glue, as it will eventually fail and pollute
- It must be easy to clean. Very important if the efficiency is to be high as
dirty filters consume more oxygen and do less work.
- The media must not clog. If the media clogs it will turn anaerobic and
convert nitrate back to nitrite.
- Easy of handling and installation. This is often a considerable problem
with medias that have small components as it can be difficult to hold them in
place or support them.
- The medias should not compact or lock together making cleaning impossible.
Most of the aqua rocks sold compact down with time and are very
difficult to clean effectively. The compacting media also creates tracking,
where the water only runs through in areas of the least restriction this
effectively stops all the media working and reduces the amount of work that can
be carried out by the filter. Tracking also increases the likely hood of the
filter blocking, and the production of anaerobic areas.
- Weight is a key factor in filter media as it, effects the tanks required
for the filter. Gravel weights about 1500kg per m3 this will have to be taken
in to account when building and installing the tank. This will also effect the
cost, the media might be cheaper but if the tank is more expensive, the light
weight media would be a better bet and easier to clean.
- The media must have a known surface area to volume ratio as it makes it
very difficult to work out the filter size if the media is random.
The first media available to pond keepers was gravel and now the choice
has become some what difficult, but there is a simple method to choosing the
media for you.Gravel
A cheap media but shows most of the
problems shown above, it compacts down , it is difficult to clean and very
This is either crushed lava rock or clinker , the
waste out of kilns, and is very difficult to clean and has a poor surface area
to volume ratio, and very difficult to clean.Lytag
clay pellet that has a large porous surface area and often floats, can be
difficult to clean and clogs easily.Flocor.
A plastic media
created for sewage farms, very good surface area to volume area and relatively
inexpensive, used on commercial fish farms. The light weight makes filter
construction easy and it is easy to handle. The only down side is that as a
loose media it traps dirty and is quite difficult to clean.
filter mat is a jumble of plastic fibres bonded together. While recommended by
fish retailers and koi experts it is a relatively poor media as it has a number
of basic problems. It is glued together and the glue eventually fails leading
to small plastic fibres floating around the filters and finding their way to
the pump which they then clog. It has a random construction and no set surface
to volume ratio. The mat is a mix of different plastics and no data is
available on the make up of it. The mat needs to be cut up and turned into
cartridges as it clogs quickly otherwise.
It is difficult to
There are a number on the market but
many have problems when it comes to the surface, as it is cheaper to produce a
media with a smooth surface so most do. This leaves the bacteria with nothing
to hang on to, this type of media will take a very long time to a mature. If
the water flow is to strong the bacteria will get washed off.Bio
One of the first effective plastic medias, can only be used in a
filter where the first stages remove all the solids as it clogs quite quickly
and they are difficult to clean. They offer a very large surface area , but the
complex nature of the design is difficult to produce and this is reflected in
the very high production cost. Best used in trickle towers.Bio
A very expensive media as it is a honeycomb of glass, made into
small sections in the shape of a doughnut. The surface area is incredibly large
,about 100 times the area per m3 of most other medias . As with most good
things it has never proven its self in ponds due to the high particle waste
levels in the water , as the surface of the media is so fine it clogs almost on
day one and is of little use after a few weeks. It is ideally suited for indoor
Ready made filters.
The simplest filters consists of a foam prefilter
on the front of the pump, these work in small ponds up to 500 litres. They
require regular maintenance and if left too long will starve the pump of water
and cause the pump to fail. Not really suitable for ponds where lots of fish
are housed due to the relatively poor filter capacity and high
The next step up in filters consist of a plastic water tank
with a spray bar spreading the water across thin sheets of foam of varying
grades this collects the particles and any dirt in the water, for the
biological action to take place a plastic ring material, which offers a large
surface area to a volume ratio, is used for bacteria to live on .
next stage is a multi-chamber filter. In this type of filter the different
filtration stages are split up this allows the use of more media and when
cleaning the biological media is left undisturbed.
The filter shown is a
simple 3 chamber model that use brushes in the first chamber for settlement and
then has two biological chambers for the bacteria, this offers the chance of
using two different medias which means that a more diverse habitats for the
bacteria to grow in, this will improve the filters efficiency. There is a
perforated tray to hold up the media and allow the water to flow underneath to
the next chamber. The settlement chamber has a drain to waste for easy
The multiple vortex filters are a newer design popular with
manufactures as they are cheaper to produce and stronger than box
The come in 3, 4 or 5 chamber styles and the media
choice is up to you. The vortexes are normally very small and the flow rates
recommended are far to quick. To get the best from these it is best to buy
larger than suggested by the manufacturers and slow the flow rate down. All
chambers have drains making cleaning easy and quick.
They are best used
as gravity fed filters other wise the pump breaks up the solids making the
collection more difficult.Fluidised filter beds
biological only and required a good prefilter to stop solids from entering the
filter system. They are very effective and very compact for what the capacity
they have. They have a set of requirements that must be provided for them to
They need a high capacity pump to start and lift the media into
The flow rate must be constant any large variation will stop
the filter from functioning. To much and the media will end up in the pond and
to little and the media settles out and will turn anaerobic.
required well oxygenated water to function as a vast amount of bacteria is held
in a very small volume.
The filter must be vertical to work correctly,
otherwise the media will settle out on one side creating an anaerobic
They must never be used with out a normal biological filter as
they are by nature unreliable, and should be considered an additional filter
not the main one.
They are effected by chemicals and die quickly if the
water flow stops as the media stops moving and sinks to the bottom, compacts
down and the bacteria run out of oxygen.
Fludised filters are normally
the domain of marine systems and commercial indoor recirculation systems but
they are always used as an additional filter because of their difficult
The materials used to build them should be stainless steel or a
tough industrial plastic, the nature of the filter with moving sand particles
makes them very hard on the filter container and they will soon wear out thin
plastic or fibreglass tanks.Trickle towers.
In the commercial
world these are the most popular as they offer the best of all filter methods,
again they offer only biological filtration.
As the media is suspended
in air, the oxygen content is about 20 times higher than in water so can
support 20 times the bacteria of a traditional submerged filter. The action of
water falling down the media exposes it the to air helping to remove ammonia
which is lost to the atmosphere as a gas and this reduces the work the filter
has to do and at the same time reduces nitrate, which in turn reduces algae
growth as there is less food for it.
The design is simple and reliable
it has few requirements and because of this is easy to set up and
The tower can be light weight as it holds no water and only has to
hold the filter media up right and contain any water as it falls through the
media. It is even possible to use thin plastic sheeting as a tower if the media
will self support. This reduces the initial out lay.
The spray bar needs
to spread the water evenly over the surface of the media this can be achieved
best with a rotating spray bar as it is constantly moving the water across the
surface and as it is not a continuous stream it does not wash the bacteria off
the media. Other methods such as a spreader plate, which is made up of a tray
with holes drilled at set distances all over the base which allows the water to
spread over the media.
The filters height needs to be around 1.2m
minimum to have any real effect.
The filters efficiency can be improved
as the fish increase in size by simply adding an air pump to the filter base so
that air is allowed to work its way up the filter and out of the top, this
increases the oxygen levels and removes any ammonia that is forced out of the
Another easy way to increase the air flow is to add an extractor
fan to the tower to pull air through the media.
The two draw backs of
trickle towers are height they need to be tall to work and they have to be
above the water level of the pond, the second is winter as they pass water
through the air they will exchange energy between them , in warm weather they
will heat the water, and in cold weather they will cool the water.
power is lost they can survive undamaged for days with out much loss of
efficiency, as the air in the tower is humid and full of oxygen.Bead
Bead filters are not a new filter, but have only recently
become available to the pond keeper.
A simple principle of using small
floating plastic beads, about 3-4mm in diameter, to act as both a
mechanical and biological filter all in one.The design makes them very compact
and easy to hide. They are also easy to clean as a simple timer that switches
the pump off for 15 minutes a day will start its cleaning cycle. A good
prefilter is needed to remove any trace of blanket weed or particles larger
than 5mm as these will otherwise clog the inlet strainer, and reduce the
through put of water.
The hour glass shape as become the trade mark of the
best models as they clean them selves with out the use of any moving parts and
have proved to be very reliable.
NOTE: The sludge valve can be replaced
with an automatic valve that opens as soon as the pump is switched off this
means the filter unit can be left to self clean and restart.
slow to start working and seem to suffer if chemicals are added to the pond.
Once established the bead filter is good filter. Use units that are larger than
recommended as they under perform compared to the manufactures
They are ideal if time is limited and you go away regular as
the self cleaning makes maintenance very simple. Starting the
All biological filters take time to processing waste
as the bacteria need to build up to sufficient numbers to start working. The
build up time can be reduced by adding a bacterial filter start,. These contain
all the nitrifing bacteria in a solution that suspends them in an inactive
mode. As so as they are added to water the bacteria start to function and
multiply. At this point all ultraviolet filters should be left off to avoid any
sterilising effect they may have.
The pond goes through new pond
syndrome where the addition of excess fish leads to a wipe out or loss of
most of the fish. This is caused by the filter not being able to break down the
fish waste as it is immature and the fish die as they are swimming in their own
When starting a new pond it should not be stocked heavily for at
least four to six weeks to avoid this effect and fish should be added in small
numbers after being quarantined first.
Testing the water in new pond
will help you to determine when it is safe to add new fish as the start up of
the filter follows a simple path.
As the first fish are added the waste
is produced as ammonia . this shows on a test kit and the level rises over time
until the bacteria appear in sufficient numbers to convert it to nitrite. The
nitrite will start to rise as the ammonia is converted and will continue until
the bacteria that convert nitrite to nitrate reach the correct numbers and the
level then drops. The nitrate level rises until it is either water changed out
or used by plants.
The speed at which the filter starts is effected by
the water temperature and the amount of oxygen there is available. The higher
the oxygen content the faster the filter will establish its self.
Every time you add fish to the pond the filter has to build up more bacteria to
help with the increased load. Often this is called shock loading and many
filters, especially small filters that are on the limit of their capacity, fail
and a complete break down leads to instantaneous waste build up and fish
deaths. So when adding a number of fish to your pond always check the levels
for a few days to make sure this does not occur.Sizing the
The creation of a biological filter is very simple and almost
any material can be used as a media from gravel to Bio-Block there is no
restriction on size or type. As long as you follow the requirements of the
bacteria it will work.
Sizing can be difficult as peoples ideas range
from 1/3rd the size of the pond to a tiny 25 litre tank.
A rough guide
is to allow 1 litre of filter media for 1 gram of food per day. A fish requires
2% of its body weight per day to live a healthy life with normal
So if your pond is to hold say 10 fish at 60cm in length, a fish
this size would weight about 3kg.
So a feed weight of 3000 x 2%= 60gms x
10 =600 gms of food per day so a biological filter holding 600 litres of media
will be fine.
It is always best to over size your filter as this formula
works well when the temperature is constant ( which it never is ) so allow
10-15% extra media to cover any variations or extra fish that find their way in
to your pond !
When building a filter allow some extra space around it
for extensions should you need it.
The last process of the aerobic filter is the
production of nitrate. Although it is not toxic until levels are very high, in
excess of 150 mg/ltr, the lower the levels the better it is for the fish, as in
their natural habitat it would be less than 1 mg per ltr.
three ways to remove nitrate effectively , with a filter , with plants and the
last with exchange resin.Denitrifing filters
denitrifing filter is a one that supports bacteria that require oxygen free
Denitrification requires a source of carbon to work,
the normal approach to this is to add either methanol or sugar as the supply,
the bacteria then break down the carbon source and nitrate to produce carbon
dioxide, nitrogen gas and water. (The chemical formula is not quite complete
but good enough to show the results in this case)
The filter tank needs to
be set up after the normal biological filter to take advantage of the low
oxygen and high nitrate content of the water exiting from here. Ideal
conditions for denitrifing bacteria are oxygen levels of 2-3mg/ltr and nitrate
of 30mg/ltr or more. If levels fall below this hydrogen sulphide will be
produced which not only smells like rotten eggs but is also toxic to the
The water needs to spend 2-4 hours passing through the denitrifing
filter for it to work( For example if the filter holds 100ltrs the flow rate
should be between 200 to 400 ltrs per hour. The outlet should return the water
to the beginning if the bio filter to reduce any nitrite that may be in the
This is a difficult system to set up and has a number of
problems that can cause major disasters if they go wrong. It is not for the
busy person, as it requires time to be spent checking it each day and back
washing it weekly to remove the sludge build up that occurs. But it uses nature
to do the work and is in general a very good method of nitrate
The most natural way to remove nitrate from
the pond water is to use plants. As plants grow they use nitrate to build the
leaves and stems that they use to collect sunlight to use to produce food for
growth as they grow they require more nitrogen.
The best way to grow
plants for vegetable filter is to grow them in a shallow water coarse that is
fed from the biological filter, where nitrate production occurs.
more the plants are cut and removed the more they grow which means they will
remove more nitrate. The plants also pick up other toxic metals and chemicals
so are also good for purifying the water as well.
The simpler the filter
design the better it works as the maintenance is also simple. The vegetable
filter should, be large enough for the flow from the bio filter and long so
that the water takes a reasonable time to travel through the filter so that the
plants have a chance to pick up the nitrate as it goes past in the water. The
filter must be free flowing as you don't want it clogging and over flow. The
plants will need regular trimming to ensure the maximum growth is achieved. The
only down side is the plants do not grow well in the winter months when the
weather is cold.Ion Exchange Resin
The last method is to use
resins that remove nitrate in exchange for another ion usually sodium chloride
( common salt ). The resin is cleaned or recharged by washing in a
salt solution of 100gms per litre water, each litre of resin needs 5ltires of
salt solution to recharge it.
This is the easiest method for removal if
the space for vegetable filter is not available , they are compact and very
good. The levels can be reduced to less than 25mg/ltr which is often lower than
the tap water, used to top the pond up.
It is difficult to say the
amount of resin required for each pond but a starting point is 1 litre of resin
for each 5000 litres of pond volume, you can always increase the amount of
resin if need be, or if it is to much there will be less cleaning.
This is probably the least understood and
the most poorly sold pond product on the market. It is sold to cure every
problem from blanket weed to parasite infestations. When in fact the standard
pond unit only cures one pond condition, green water.The idea is to pass the
green water through a bath of UV light which then effectively stops the algae's
ability to reproduce. The water then clears and all is well, as long as the
bulb lasts, then the pond goes green again if the pond plants have not grown
enough to compete with the algae.
All UV systems work by
circulating pond water around the UV bulb, whether it be the older double ended
bulbs or newer single ended type, which is encased within a quartz sleeve which
protects the bulb from contact with the water. The outer case directs the water
along the out side of the sleeve , ideally in a spiral pattern this allows for
the maximum contact time with the UV light, and no water can escape the light
therefore giving a better kill rate and higher efficiency. (Although there is
only one manufacturer producing such a unit with the spiral flow built
UV light is absorbed very quickly and there is a surprising amount of
factors which effect the UVs efficiency.
- The speed at which the water flows through the unit, to fast and the
contact time will be to short, and to slow and the kill rate will be higher but
the algae will grow faster than it can be killed.
- Suspended particles in the water, the more debris the water is carrying
the more light will be absorbed and the less the work the UVC will do.
- Dirt on the quartz sleeve. If the sleeve is dirty the light can not get out
- Chemicals in the water. Any dyes that are used as treatments will effect
the lights ability to travel through the water.
- Poorly designed UV units , This is the biggest cause of UVC not working.
There are a number of designs which are very inefficient and should be avoided.
Any unit that suspends the bulb above the water is asking for trouble, water
can splash on hot the bulb and electric. Units that pump the water through a
quartz sleeve are very poor as the amount of UV light that gets from the bulb
to the water is so small ( about 10% or less). A properly designed sleeved UV
bulb will put about 99% of its output into the water, therefore giving the most
efficient and safest use of UV output.
There are two types of Ultraviolet systems that are available, - the
standard pond Ultraviolet Clarifier (UVC)PIC 15 and the Ultraviolet Steriliser
The design of the UVC is very similar but has a number of
factors that separate it from the UV sterilisers. A large design difference
between them is the gap between the quartz and the outer casing. The UVC is
only designed to kill algae so the gap is large, up to 4cm, and allows a high
through put and low resistance to small pumps.
The out put near the
casing is very low as the water absorbs the UV light, this means the casing
material can be thinner than on a UVS.
The gap on a UVS can be as small
as 0.5cm, in most cases it is about 1-1.5cm this means the UV level is very
much higher and at this dose even bacteria, viruses and parasites can be
killed, this dose also takes its toll on the casing as the UV levels are very
high therefore UVS are normally made from industrial plastic materials or
stainless steel. The flow rate has to drop to very low volumes, as the contact
time required to kill bacteria etc is far longer than to kill algae, and this
means to service a pond many more UVS would be needed against a UVC just to
control algae. This explains the higher cost of sterilisers but they do a
totally different job to a Clarifier.
UVS are best put after filters in
the return ( if possible) as they work better if the water is free from
floating particles, where as UVCs will work on either the dirty or clean side
of the filter as the UV dose is less important.
One of the most
important factors effecting the UV units are the bulbs they have a relatively
short life between 8000 to 9000 hours continuous use.
It is often the
aquatic retailers that tell pond keepers that the more UVCs on a pond the
better, this is untrue. Once the water is clear it is pointless to have more
and more UVCs as they are unable to kill bacteria and parasites as the UV dose
is too low and the contact time to short.
There are a few myths about UV
systems in general;
1) They sterilise the pond and lower the
fishs immunity to disease
This is not true as it would be
impossible to create a sterile environment in the open area of a garden, and
UVCs do not sterilise at all . UVS only keep water borne diseases under
control, not any thing on the fish themselves.
2) UVCs control
This is unlikely as the blanket weed does not pass
through the UV chamber.
3) UVs effect the water chemistry
and this can kill the fish
No tests have been done that prove this and
I have never seen any thing to even suggest this could happen.( One of the main
reasons UV units are used to sterilise drinking water is that they do not
effect the water chemistry).
4) If the bulb is glowing it is still
This is also untrue, the reason the manufactures of the bulbs
give you a life for optimum use is because they know it lasts this long ! and
as the UV light is invisible the fact that it glows and you can see it is not
There is no doubt that Ultraviolet filters play an important
part in filtering the pond but, if the biological filters are the correct size
and the pond is not over stocked there is no reason that the UV cannot be used
for the start of the season and then turned off as the filters start to
There is always differences of opinion when it comes
to filtration and there will always be the pond which survives with nothing and
is perfect unfortunately these are few and far between. There is
nothing worst than a green pond and with modern pumps and filters there is no
excuse for it. If selecting a ready made unit always read the instructions as
they will always say the filter will treat a huge pond but if the factors they
mention are taken in to account the filters capacity diminishes very quickly
and a 10000 litre filter soon only filters a 2500 litre pond and once you have
used it taking it back will be difficult.
You can not over filter a pond
as it this means is that the filter will take longer between cleaning as it
will clog slower if it has a larger surface area. The filter will not be
effected by a sudden increases in fish population as the excess filter media
will just kick in with out a struggle. This excess filter capacity is a good
back up and if the budget will stretch to it is a worth while investment.